Document how to schedule periodic messages from an actor to itself. #2513

akka-docs/rst/scala/code/docs/pattern/SchedulerPatternSpec.scala

@@ -0,0 +1,98 @@
+/**
+ *  Copyright (C) 2009-2012 Typesafe Inc. <http://www.typesafe.com>
+ */
+
+package docs.pattern
+
+import language.postfixOps
+
+import akka.actor.{ Props, ActorRef, Actor }
+import scala.concurrent.util.duration._
+import scala.concurrent.util.{ FiniteDuration, Duration }
+import akka.testkit.{ TimingTest, AkkaSpec, filterException }
+import docs.pattern.SchedulerPatternSpec.ScheduleInConstructor
+
+object SchedulerPatternSpec {
+  //#schedule-constructor
+  class ScheduleInConstructor extends Actor {
+    import context._
+
+    val tick = system.scheduler.schedule(500 millis, 1000 millis, self, "tick")
+    //#schedule-constructor
+    var target: ActorRef = null
+    def this(target: ActorRef) = { this(); this.target = target }
+    //#schedule-constructor
+
+    override def postStop() = tick.cancel()
+
+    def receive = {
+      case "tick" ⇒
+        // do something useful here
+        //#schedule-constructor
+        target ! "tick"
+      case "restart" ⇒
+        throw new ArithmeticException
+      //#schedule-constructor
+    }
+  }
+  //#schedule-constructor
+
+  //#schedule-receive
+  class ScheduleInReceive extends Actor {
+    import context._
+    //#schedule-receive
+    var target: ActorRef = null
+    def this(target: ActorRef) = { this(); this.target = target }
+    //#schedule-receive
+
+    override def preStart() =
+      system.scheduler.scheduleOnce(500 millis, self, "tick")
+
+    // override postRestart so we don't call preStart and schedule a new message
+    override def postRestart(reason: Throwable) = {}
+
+    def receive = {
+      case "tick" ⇒
+        // send another periodic tick after the specified delay
+        system.scheduler.scheduleOnce(1000 millis, self, "tick")
+        // do something useful here
+        //#schedule-receive
+        target ! "tick"
+      case "restart" ⇒
+        throw new ArithmeticException
+      //#schedule-receive
+    }
+  }
+  //#schedule-receive
+}
+
+class SchedulerPatternSpec extends AkkaSpec {
+
+  def testSchedule(actor: ActorRef, startDuration: FiniteDuration,
+                   afterRestartDuration: FiniteDuration) = {
+
+    filterException[ArithmeticException] {
+      within(startDuration) {
+        expectMsg("tick")
+        expectMsg("tick")
+        expectMsg("tick")
+      }
+      actor ! "restart"
+      within(afterRestartDuration) {
+        expectMsg("tick")
+        expectMsg("tick")
+      }
+      system.stop(actor)
+    }
+  }
+
+  "send periodic ticks from the constructor" taggedAs TimingTest in {
+    testSchedule(system.actorOf(Props(new ScheduleInConstructor(testActor))),
+      3000 millis, 2000 millis)
+  }
+
+  "send ticks from the preStart and receive" taggedAs TimingTest in {
+    testSchedule(system.actorOf(Props(new ScheduleInConstructor(testActor))),
+      3000 millis, 2500 millis)
+  }
+}

akka-docs/rst/scala/howto.rst

@@ -111,6 +111,34 @@ This is where the Spider pattern comes in."
 
 The pattern is described `Discovering Message Flows in Actor System with the Spider Pattern <http://letitcrash.com/post/30585282971/discovering-message-flows-in-actor-systems-with-the>`_.
 
+Scheduling Periodic Messages
+============================
+
+This pattern describes how to schedule periodic messages to yourself in two different
+ways.
+
+The first way is to set up periodic message scheduling in the constructor of the actor,
+and cancel that scheduled sending in ``postStop`` or else we might have multiple registered
+message sends to the same actor.
+
+.. note::
+
+   With this approach the scheduler will be restarted with the actor on restarts.
+
+.. includecode:: code/docs/pattern/SchedulerPatternSpec.scala#schedule-constructor
+
+The second variant sets up an initial one shot message send in the ``preStart`` method
+of the actor, and the then the actor when it receives this message sets up a new one shot
+message send. You also have to override ``postRestart`` so we don't call ``preStart``
+and schedule the initial message send again.
+
+.. note::
+
+   With this approach we won't fill up the mailbox with tick messages if the actor is
+   under pressure, but only schedule a new tick message when we have seen the previous one.
+
+.. includecode:: code/docs/pattern/SchedulerPatternSpec.scala#schedule-receive
+
 Template Pattern
 ================
 
@@ -127,4 +155,3 @@ This is an especially nice pattern, since it does even come with some empty exam
    Spread the word: this is the easiest way to get famous!
 
 Please keep this pattern at the end of this file.
-